Rocks and Minerals B/C

[EmeraldChocolate]

can someone help with how do you tell the difference between quartz(crystal), selenite, albite, calcite, and other colorless minerals with the same crystal structure?
while we're on that topic, i really would appreciate help on the difference between fluorite and beryl and topaz
and between the diorite, gabbro, and granite
kaolinite, diatomite, and chalk
you guys all seem like such experts, and i'm really new to this
help?

[gneissisnice]

can someone help with how do you tell the difference between quartz(crystal), selenite, albite, calcite, and other colorless minerals with the same crystal structure?
while we're on that topic, i really would appreciate help on the difference between fluorite and beryl and topaz
and between the diorite, gabbro, and granite
kaolinite, diatomite, and chalk
you guys all seem like such experts, and i'm really new to this
help?

They have VERY different crystal structures.

Quartz forms six-sided crystals that almost always terminate at one end (come to a point). It has a hardness of 7, which means that it scratches glass.

Selenite can look similar to calcite, but the main identifier is that selenite is very soft. Gypsum is a 2 and calcite is a 3. It's only a difference of 1 on the Mohs scale, but it's a huge difference because a fingernail has a hardness of 2.5. You can scratch selenite with your fingernail but not calcite.

Calcite has a ton of unique identifiers. Its rhombohedral crystal shape is a dead giveaway. Remember, it looks like a prism made from parallelograms, it is NOT a cube (some people mix them up). Calcite also fizzes in acid. You might not always get acid at a competition, but some event writers will tell you if the sample fizzes or not. On that note, any time they tell you that a mineral fizzes in acid, that narrows down the choice significantly. Fizzing in HCl means that it must be a carbonate mineral, and of those, the only ones on your list are calcite, aragonite, dolomite, azurite, and malachite, and they're all easily distinguishable. Calcite also exhibits double refraction. If you get a transparent sample, put it on top of your test and look through it. You'll see that it splits the words into two because it is birefringent.

The above three minerals tend to be transparent or translucent, but albite is typically opaque. You can get opaque calcite though, but albite is much harder, with a hardness of 6 (it should just scratch glass). Albite, as a feldspar, is most easily identified by its cleavage. In any feldspar sample, you should see two planes of cleavage that make a 90 degree angle. In practical terms, you should see two smooth, flat, shiny sides that meet together while the other sides are more rough.

Out of the next three, fluorite is by far the softest. With a hardness of 4, it won't scratch glass. Fluorite also has 4 planes of cleavage, and while you don't always get nice samples that show it on the sides, if you look closely in a translucent/transparent sample, you should see the cleavage planes inside the mineral. Fluorite is often green or purple, but it can be other colors too. And of course, fluorite glows under UV light.

Beryl and topaz can be a bit tricky. Both have a very high hardness, so they scratch glass with ease. Beryl has a hexagonal crystal structure and will generally appear as a six-sided column. Topaz is very lustrous and if you get a transparent sample, you can see kind of a rainbow, shimmery effect sometimes. It's hard to describe, but that's how I tell the difference.

As for the igneous rocks, all igneous rocks are classified based on two criteria: their composition (color) and texture (crystal size). Diorite, gabbro, and granite all have a coarse (large crystals) texture, but they vary based on composition. Granite is felsic, which means that it contains light colored minerals like potassium feldspar and quartz. Pink or clear minerals are a dead giveaway that the sample is granite. Diorite is intermediate. It will have a mix of light and dark minerals, mostly gray with spots of white or black. Gabbro is mafic, so it will be black, possibly with some green in it.

Kaolinite is a mineral while diatomite and chalk are rocks. They can be difficult to tell apart. Kaolinite is a clay mineral, and often has some brown in it, but not always. Diatomite and chalk are almost impossible to distinguish in hand sample. Luckily, the difference isn't too bad. Both are made up of the skeletons of once-living organisms, but the composition differs greatly. Diatomite is made from diatoms, which have a silica skeleton, so it is mostly made up of SiO2. Chalk is made of coccolithophores, which have a calcite skeleton, so it is made of CaCO3. That means that chalk (being a form of limestone) will fizz in acid and diatomite won't. If the event writer tells you that the sample fizzes in acid, it is definitely chalk.

Hope that helped.

[vxpvm]

Are we allowed to use hardness picks when identifying minerals? I know on the rules sheet it isn't on the list of things to bring, but will we generally be given hardness picks at stations or have you guys had to do things like id between beryl and apatite without hardness?

no tools like that are not allowed because most of the samples are museum quality from private collections and that information is usually given.

Actually, it depends. At a competition once we were given the hardness picks, but usually I haven't seen them. The whole beryl/apatite/annoying hard minerals difference I still find annoying...some say color and whatnot, but idk fully myself.

Okay, cool. Thank you guys so much! I've been visiting museums to try and get a feel for minerals outside of the world of Google images, and identifying some minerals without hardness was one of my worries. Does anybody know how to tell the difference between aragonite and calcite? Because it looks like they can both occur in basically every form possible and they both fizz under HCl so ...

[gneissisnice]

Are we allowed to use hardness picks when identifying minerals? I know on the rules sheet it isn't on the list of things to bring, but will we generally be given hardness picks at stations or have you guys had to do things like id between beryl and apatite without hardness?

no tools like that are not allowed because most of the samples are museum quality from private collections and that information is usually given.

Actually, it depends. At a competition once we were given the hardness picks, but usually I haven't seen them. The whole beryl/apatite/annoying hard minerals difference I still find annoying...some say color and whatnot, but idk fully myself.

Okay, cool. Thank you guys so much! I've been visiting museums to try and get a feel for minerals outside of the world of Google images, and identifying some minerals without hardness was one of my worries. Does anybody know how to tell the difference between aragonite and calcite? Because it looks like they can both occur in basically every form possible and they both fizz under HCl so ...

Aragonite and calcite are polymorphs of each other, meaning that they're both CaCO3 but they have different crystal structures.

Look at the crystal shapes. If it's a rhombus, it's definitely calcite. If it looks like a stubby column, it's aragonite. They pretty much have the same properties, so you need to go by crystal structure.

[inluvwyth_WANTED]

I just wanted to know what kind of tests you guys have gotten before? This event is not new, in its last year, but I'm just curious as to the different tests you guys have seen. Please reply!

[vxpvm]

I just wanted to know what kind of tests you guys have gotten before? This event is not new, in its last year, but I'm just curious as to the different tests you guys have seen. Please reply!

Woah wait, are you sure it's the last year? Wow that sucks... I just ordered the rock kit haha. Well the tests that I've had or have been told about were all station based, so there would be a bunch of tables set up in a room with different minerals on each, and you'd have do identify them. Usually it isn't just one mineral or rock on a table, it'll be like four clear minerals or four metamorphic rocks or something. And then there are questions about the specimens, of course.

[darkdeserthighway]

I just wanted to know what kind of tests you guys have gotten before? This event is not new, in its last year, but I'm just curious as to the different tests you guys have seen. Please reply!

Woah wait, are you sure it's the last year? Wow that sucks... I just ordered the rock kit haha. Well the tests that I've had or have been told about were all station based, so there would be a bunch of tables set up in a room with different minerals on each, and you'd have do identify them. Usually it isn't just one mineral or rock on a table, it'll be like four clear minerals or four metamorphic rocks or something. And then there are questions about the specimens, of course.

Yeah, it's in its last year and the tests are usually run in stations, with a few questions per station.

[sofossils]

I was recently taking a practice test and confused amazonite with beryl. I was wondering if anyone knew the difference between these two minerals. Any help in distinguishing them would be greatly appreciated.

[darkdeserthighway]

I sent you a PM

[hexagonaria]

Simple question: which crystal system does quartz belong to?
The Audubon guide says hexagonal, which is what I always thought, but everywhere online it says trigonal. Anyone else have this problem?